This invention relates to a sodium leakage detection system and a method of controlling the same, the system detecting a leakage of sodium from piping and/or instruments inserted therein in a fast breeder reactor or other installations where a great amount of liquified sodium metal is used.
In fast breeder reactors now in practical use and still under development, liquified sodium metal is usually used as coolant. If such coolant leaks through cracks formed in a pipe or an instrumental part and is exposed to the atmosphere, the chemically active property of sodium offers a positive danger of fire breaking out. Further, corrosive substances produced due to the reaction of the leaked sodium on oxygen or moisture in the external atmosphere, accelerate the process of corrosion taking place in pipes or instrumental parts. And the enhanced corrosion may lead to an accidental, large-scale leakage and in an extreme case to a Loss of Coolant (LOC) accident. To prevent such as accident from taking place, cable type or spark-plug type leakage detectors are employed in the coolant circulation system. However, these types of leakage detectors operate on the phenomenon that the actually leaked sodium depositing on the detectors changes electrical conductivity in the detectors. Therefore, the detection is not possible unless a relatively large amount of sodium leaks. Needless to say, it is preferable to be able to detect a leakage while it is of a small quantity. For the purpose of detecting a leakage in the earlier stage, a sodium ionization detector (Japanese Patent Application No. 124172/78, U.S. patent application No. 83658) and a pressure difference detector (Japanese Patent Application No. 156462/78) have been proposed and are considered promising. In the application of these detectors, the gas surrounding the pipe or instrument of which the leakage is detected, is sampled and the sample gas is led to the detector so that sodium vapor or aerosol is detected. In the case of the pressure difference detector, a membrane filter is placed in the flow of gas and as the sodium aerosol suspended in the gas fill the micropores of the membrane filter, the difference between the gas pressures in front of and behind the filter increases. Accordingly, a leakage of sodium can be identified by detecting the pressure difference.
As described above, the surrounding gas sampling system is found more preferable for the detection of sodium leakage in the earlier stage, that is, while the leakage is still small. Further, in the case of a fast breeder reactor where several tens to several hundreds of leakage monitoring points are separately selected, the sampling system to be employed should preferably be of such a type as disclosed as an automatic sampling device in the U.S. Pat. No. 3,245,269 specification, in which a single leakage detector is provided for a plurality of leakage monitoring points and the desired samples of surrounding gas are selectively conducted into the detector through a change-over operation.
Even in this case, there are still some problems remaining to be solved. One of those problems is that since the gas samples from different monitoring points are successively taken into the detector, the present sample gas tends to be contaminated by the residual of the previous sample gas. Another problem is to exactly locate the position where an abnormal condition is detected in the leakage detection system and also to exactly judge whether the abnormal condition is due to sodium leakage or to the malfunction of detecting means.